Inspection and Testing6
Insulation tests
Testing insulation resistance
A low resistance between phase and neutral conductors, or
from live conductors to earth, will result in a leakage current. This current
could cause deterioration of the insulation, as well as involving a waste of
energy which would increase the running costs of the installation. Thus, the
resistance between poles or to earth must never be less than half of one meg ohm
(0.5 M Ohms) for the usual supply voltages. In addition to the leakage current
due to insulation resistance, there is a further current leakage in the
reactance of the insulation, because it acts as the dielectric of a capacitor.
This current dissipates no energy and is not harmful, but we wish to measure the
resistance of the insulation, so a direct voltage is used to prevent reactance
from being included in the measurement. Insulation will sometimes have high
resistance when low potential differences apply across it, but will break down
and offer low resistance when a higher voltage is applied. For this reason, the
high levels of test voltage shown in {Table 8.8} are necessary. {8.7.1} gives test
instrument requirements.
Before commencing the test it is important
that:
1. - electronic equipment which could be damaged by
the application of the high test voltage should be disconnected. Included in
this category are electronic fluorescent starter switches, touch switches,
dimmer switches, power controllers, delay timers, switches associated with
passive infrared detectors (PIRs), RCDs with electronic operation etc. An
alternative to disconnection is to ensure that phase and neutral are connected
together before an insulation test is made between them and
earth.
2. - capacitors and indicator or pilot lamps must
be disconnected or an inaccurate test reading will result.
Table 8.8 - Required test voltages and minimum
resistance
| ||
| Nominal circuit voltage |
Test voltage
(V) |
Minimum insulation resistance
(M Ohms) |
| Extra-low voltage circuits supplied from a safety
transformer |
250
|
0.25
|
| Up to 500 V except for above |
500
|
0.5
|
| Above 500 V up to 1000 V |
1000
|
1.0
|
| The insulation resistance tester must be capable of
maintaining the required voltage when providing a steady state of current of
1mA. | ||
The test to earth {Fig 8.10} must be carried out on the
complete installation with the main switch off, with phase and neutral connected
together, with lamps and other equipment disconnected, but with fuses in,
circuit breakers closed and all circuit switches closed. Where two-way switching
is wired, only one of the two strapper wires will be tested. To test the other,
both two-way switches should be
 |
Fig 8.10 - Insulation test to
earth
 |
Fig 8.11 - Insulation tests between
poles
operated and the system retested. If desired, the
installation can be tested as a whole, when a value of at least 0.5 M Ohms
should be achieved, see {Fig 8.10}. In the case of a very large installation
where there are many earth paths in parallel, the reading would be expected to
be lower. If this happens, the installation should be subdivided and retested,
when each part must meet the minimum requirement.
The tests to earth {Fig 8.10} and between poles {Fig 8.11}
must be carried Out as indicated, with a minimum acceptable value for each test
of 0.5 M Ohms. However, where a reading of less than 2 M Ohms is recorded for an
individual circuit, (the minimum value required by the Health and Safety
Executive), there is the possibility of defective insulation, and remedial work
may be necessary. A test result of 2 M Ohms may sometimes be unsatisfactory. If
such a reading is the result of a re-test, it is necessary to consult the data
from previous tests to identify deterioration. A visual inspection of cables to
determine their condition is necessary during periodic tests; perished
insulation may not always give low insulation readings
As indicated above, tests on SELV and PELV circuits are
carried out at 250 V. However tests between these circuits and the live
conductors of other circuits must be made at 500 V. Tests to earth for PELV
circuits are at 250 V, whilst FELV circuits are tested as LV circuits at 500
V. Readings of less than 5 M will
require further investigation.
Tests of non-conducting floors and walls
Where protection against indirect contact is provided by a
non-conducting location, the following requirements apply.
1. - there must be no protective
conductors
2. - if socket outlets are used they must not have an
earthing contact
3. - it should be impossible for any person to touch
two exposed conductive parts at the same
time
4. - floors and walls must be
insulating.
To test this last item and so to make sure that the floors
and walls are non-conducting, their insulation has to be tested.
The requirements are shown in {Fig 8.12}, the electrodes used
for making contact
 |
Fig 8.12 - Insulation test of floors and
walls for non-conducting location
with floors and walls being a special type which are pressed
onto the surface with a force of not less than 750 N (77 kg or 169 lb) for
floors or 250 N (26 kg or 56 lb) for walls. The resulting insulation resistance
of not less than three points on each surface, one of which must be between 1 m
and 1.2 m from an extraneous conductive part (if there is one), measured at 500
V, must not be less than 0.5 MOhms. Attention is drawn to the natural reduction
in the insulation resistance of a surface as humidity increases. Where
insulation is applied to an extraneous conductive part to provide a
non-conducting location, this insulation must be tested with an alternating p.d.
of 2 kV. In normal use, the leakage current should not exceed I mA.
Tests of barriers and enclosures
Throughout the Regulations reference is made to the use of
barriers and enclosures to prevent contact with live parts (direct contact). If
manufactured equipment's comply with the British Standards concerned, they will
not need further testing, but where barriers and enclosures have been provided
during erection of the installation, they must be tested. Full details of the IP
classification system will be found in {Table 2.1}, but the two most
common tests are for:
1. - IP2X - no contact can be made with a probe 12 mm
in diameter and 80 mm long
2. - IP4X - no contact can be made with a rod of
diameter 1 mm.
Tests for electrical separation of circuits
This section is concerned with tests necessary to ensure the
safety of separated extra-low voltage (SELV), protective extra-low voltage
(PELV) and functional extra-low voltage (FELV) circuits which are explained in
{7.16}. In general,
the requirement is a thorough inspection to make sure that the source of low
voltage (most usually a safety isolating transformer) complies in all respects
with the British Standard concerned, followed by an insulation test between the
extra-low voltage and low voltage systems. The test is unusual in that a 500 V
dc supply (from an insulation resistance tester) must be applied between the
systems for one minute, after which the insulation resistance must not be less
than 5 MOhms for SELV or PELV systems, or 0.5 MOhms for FELV systems. A further
test at 3750 V dc for one minute is passed if no flashover occurs. This test in
particular can be dangerous, and special care should be taken.
For SELV and FELV circuits, additional inspection must ensure
that the low voltage requirements (not exceeding 50 V ac or 120 V dc) are met.
If the voltage exceeds 25 V ac or 70 V dc (60 V ripple-free), barriers and
enclosures must be tested to IP2X (see {Table 2.1})and a 500 V dc
insulation test applied for one minute between the live conductors and metal
foil wrapped round the insulation should produce a result of at least 0.5
MOhms
When insulation testing on electrically isolated circuits or
on equipment which might be damaged by the test voltage, phase and neutral must
be connected together and the test applied between them and earth.
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